Oil-based sludges of various types and consistencies are commonly generated as waste streams during oil or other hydrocarbon production processes. These sludges arise during well tests and initial production, as a by-product waste stream of hydrocarbon production, and as tank bottom sediments. The basic components of sludges are hydrocarbon oils of various consistencies, water, and solids of an inorganic and organic nature. Oil-based sludge typically refers to a complex water-in-oil emulsion stabilized by salts of organic compounds and fine solids. The oil phase contains a complex mixture of hydrocarbons of various consistencies including waxes and asphaltenes which may be solid or semi-solid at ambient temperature.
The chemistries of oil-based sludges and the relative proportions of the oil, water, and solid phases of sludges vary greatly and can change over time. To dispose of the waste, sludge is often stored in open pits where it may be left for considerable time before being treated. During such aging periods, the sludge or “pit sludge” undergoes overall chemical composition changes due to the effects of weathering, including: volatilization of lighter hydrocarbons; temperature induced crosslinking of hydrocarbons; addition of rain water; and, invariably, the introduction of a variety of other contaminants, particulates, and debris. In addition to a variable complex chemistry, oil-based sludge typically has a high solids content. Sludge solids normally include both high density and low density solids. High density solids, i.e., high gravity solids, may be large solids introduced into the drilling fluid during the drilling of a formation (e.g., formation solids, drill bits, etc.) or other solids that are relatively dense such as barite or hermatite. While low density solids, i.e., low gravity solids, are those solids within the sludge that have a lower density or are relatively small fine solids (e.g., entrained solids such as sand).
Currently, treatment of sludge is a major operational cost for producers. Sludge is collected, stored, and then disposed of in tanks or delivered to a sludge pit. One challenge of sludge treating systems is that the recovery of marketable oil from the sludge is generally not cost-effective and thus not commercially viable. Due to wide variability in sludge composition, different sludge processing systems may be needed to optimize the processing of sludge for recovering oil of sufficient quality in a cost efficient manner. The quality of oil is frequently characterized by its Basic Sediment and Water (BS&W) content, in vol. %. The current marketable BS&W of recovered oil is less than about 2 vol. %. Furthermore, it is desirable to treat pit sludge to reduce the risk of contamination of the surrounding pit area, in accordance with increasingly strict environmental regulations, as well as decrease the overall waste volume, and ultimately to permit pit closure.